Staphylococcus aureus is one of the most common causes of serious and life-threatening infections in the world. Due to their increasing frequency, increasing drug-resistance, and high morbidity and mortality, a vaccine to prevent S. aureus infections would have an enormous impact on global and US health. We have isolated a gene product, ALS3, that encodes a surface adhesin utilized by the fungus Candida albicans to adhere to human cells. We recently made the exciting discovery that candidal Als3p has striking three dimensional structural homology (>90%) with surface adhesins expressed by S. aureus. Remarkably, our preliminary studies confirm that vaccination of mice with the recombinant N-terminus of Als3p (rAls3p-N) mediates significant cross-kingdom protection against S. aureus, and markedly improves the survival of mice infected with an otherwise fatal inoculum of methicillin-resistant Staphylococcus aureus (MRSA). To date, the precise immunological requisites for host protection against S. aureus infections have been relatively unexplored. Lack of knowledge about requisites for host protection has likely contributed to recent failures of active vaccine strategies targeting S. aureus. We hypothesize that rAls3p-N induces protection against S. aureus by stimulating a coordinated Type 1-Type 2 immune response that optimizes opsonophagocytic activity. To test this hypothesis and to define the general immunological requisites for host protection against S. aureus, we will: 1) determine the impact of rAls3p-N vaccination on murine Type 1/Type 2 immunopolarization during S. aureus infection;2) determine the relative contributions of antibody and cell-mediated immune function to murine host protection against S. aureus;3) elucidate the end-effector mechanisms involved in vaccine-induced protection of mice against S. aureus;and 4) define the breadth of activity of the rAls3p-N vaccine against multiple strains of Staphylococcus and in inbred and outbred mice. Due to the ongoing increase in drug-resistant S. aureus infections, there is a desperate need for a successful active vaccine against this ubiquitous pathogen. The studies proposed will define the requisites for host protection against S. aureus, which is a crucial step in optimizing rAls3p-N vaccine efficacy. The insights into fundamental immunological mechanisms of protection against S. aureus will also be of extreme importance to the development of other vaccine candidates targeting S. aureus in the future.